Actual Apiary user here. Couldn't care less about the mocking up part. I use it because it's a fantastic tool for writing good-looking API documentation quickly.
Also a user. Another big use case is developing for mobile against the planned API while the API is still being developed.
And for systems where the API is provided by several different apps rather than just one, it's nice to have a centralized spec for everyone to conform to. Eventually it's supposed to test your api against your spec, which would be nice.
And for systems where the API is provided by several different apps rather than just one, it's nice to have a centralized spec for everyone to conform to. Eventually it's supposed to test your api against your spec, which would be nice.